Project 1

Process ID (pid)

Synopsis

#include <unistd.h>

pid_t getpid(void) – returns the pid of the currently running process.

pid_t getppid(void) – returns the pid of the parent of the currently running process.

POSIX

fork System CallCreates child process by copying

parent’s memory image

SYNOPSIS

#include <unistd.h>

pid_t fork(void)

POSIX

fork return values

• Returns 0 to child

• Returns child PID to parent

• Returns –1 on error

Fork AttributesChild inherits:• Parent’s memory image • Most of the parent’s attributes including

environment and privilege.• Some of parent’s resources such as open

files.

Child does not inherit:• Parent pid.• Parent time clock (child clock is set to 0).

fork Example#include <stdio.h>#include <unistd.h>#include <sys/types.h>void main(void){ pid_t childpid;

childpid = fork()) if(childpid == -1 {

perror(“failed to fork”);return 1; }

if(childpid == 0) { fprintf(stderr, "I am the child, ID = %ld\n", (long)getpid()); /* child code goes here */ } else if (childpid > 0) { fprintf(stderr, "I am the parent, ID = %ld\n", (long)getpid()); /* parent code goes here */ wait(NULL); }}

Second Fork Example… ; // parent codeif ((childpid1 = fork())<0) perror(“child1 fork failed”);else if (childpid1 == 0) {

… ; // child1 code }else if (childpid1 > 0) { … ; // more parent code if ((childpid2 = fork())<0) perror(“child2 fork failed”); else if (childpid2 == 0) { … ; // child2 code } else if (childpid2 > 0) { … // yet more parent code wait(NULL); // note that two waits are necessary wait(NULL); } }… ; // yet even more parent code

File Descriptors

• open

• read

• write

• close – all use file descriptors

• ioctl

File Pointers

• fopen

• fscanf

• fprintf

• fread – all use file pointers

• fwrite

• fclose

Handles

Handle is a generic term for both file descriptors and file pointers.

File Pointers

File pointer handles for standard input, standard output and standard error are:

• stdin• stdout – defined in stdio.h• stderr

File pointer is a pointer to a file structure.

File Descriptor

File descriptor handles for standard input, standard output and standard error are:

• STDIN_FILENO

• STDOUT_FILENO

• STDERR_FILENO

File descriptor in the case of open is a pointer to a file descriptor table

open#include <sys/types.h>

#include<sys/stat.h>

#include <fcntl.h>

int open(const char *path, int oflag, …);

oflag values: O_RDONLY, O_WRONLY, O_RDWR, O_APPEND, O_EXCL, O_NOCTTY, O_NONBLOCK, O_TRUNC

Open System Callmyfd = open (“/home/ann/my.dat”, O_RDONLY);

Open system call sets the file’s status flags according to the value of the second parameter, O_RDONLY which is defined in fcntl.h

If second parameter sets O_CREATE, a third parameter specifies permissions:

int fd;

mode_t fd_mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;

if((fd=open(“/home/ann/my.dat”, O_RDWR | O_CREAT, fd_mode))

== -1)

perror(“Could not open /home/ann/my.dat”);

File Permissions

user group other

r w x r w x r w xbit 8 bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0

File PermissionsPOSIX.1 File Modes

Symbol Meaning

S_IRUSR

S_IWUSR

S_IXUSR

S_IRWXU

Read permission bit for owner

Write permission bit for owner

Execute permission bit for owner

Read, write, execute for owner

S_IRGRP

S_IWGRP

S_IXGRP

S_IRWXG

Read permission bit for group

Write permission bit for group

Execute permission bit for group

Read, write, execute for group

S_IROTH

S_IWOTH

S_IXOTH

S_IRWXO

Read permission bit for others

Write permission bit for others

Execute permission bit for others

Read, write, execute for others

S_SUID

S_ISGID

Set user ID on execution

Set group ID on execution

File Descriptor LayoutFile

Descriptor

Table

[0]

[1]

[2]

[3]

…

System File

Table

In-Memory

Inode Table

myfd

3

user program area

kernel area

B

File Descriptor Table

• A file descriptor such as myfd in the previous example is just an entry in a file descriptor table.

• A file descriptor is just an integer index.

System File Table• Contains an entry for each active open file.• Shared by all processes in the system.• Contains next read position.• Several entries may refer to the same

physical file.• Each entry points to the same entry in the “in

memory inode table”.• Without the “in memory inode table”, if cycle

time were 1 second, time to access an inode would be 11 days.

• On fork, child shares system file table entry, so they share the file offset.

File Pointers and Buffering

• File pointer is a pointer to a data structure in the user area of the process.

• The data structure contains a buffer and a file descriptor.

FILE *myfp;if((myfp = fopen(“/home/ann/mydat”, “w”)) == NULL)

fprintf(stderr, “Could not fopen file\n”);Else

fprintf(myfp, “This is a test”);

File Pointer LayoutFile

Descriptor

Table

[0]

[1]

[2]

[3]

…

System File

Table

In-Memory

Inode Table

myfp

user program area

kernel area

/usr/s/al/al.dat

“This is the al.dat file”

3

B

Disk Buffering

• fprintf to disk can have interesting consequences

• Disk files are usually fully buffered.• When buffer is full, fprintf calls write with file

descriptor of previous section.• Buffered data is frequently lost on crashes.• To avoid buffering use fflush, or call a program

called setvbuf to disable buffering.

Terminal I/O Buffering

• Files are line buffered rather than fully-buffered (except for standard error which is not buffered).

• On output, the buffer is not cleared until the buffer is full or until a newline symbol is encountered.

Open my.dat Before ForkParent PDT

[0] std in

[1] std out

[2] std error

[3] my.dat

[4]

Child PDT

[0] std in

[1] std out

[2] std error

[3] my.dat

[4]

System File Table (SFT)

std in

std out

std error

my.day (parent and child)

Open my.dat After forkParent FDT

[0] standard in

[1] standard out

[2] standard error

[3] my.dat

[4]

Child FDT

[0] standard in

[1] standard out

[2] standard error

[3] my.dat

[4]

System File Table (SFT)

std in

std out

std error

my.dat (parent)

my.dat (child)

Filter

• Reads from standard input• Performs a transformation• Outputs result to standard output• Write error messages to standard error• Common useful filters are: head, tail,

more, sort, grep, and awk.• If no input file is specified, cat behaves like

a filter.

Redirection

• Modifies the file descriptor table

• > on the command line is redirection of standard output

• < on the command line is redirection of standard input

cat > my.file

cat > my.file

[0]

[0]

[1]

[2]

[2]

[1]

my.file

[0] standard input

[1] standard output

[2] standard error

[0] standard input

[1] my.file

[2] standard error

dup2

int dup2(int filedes1, int filedes2)

• First field, filedes1, is a file descriptor. It can be defined through open or through named pipe.

• Second field can be STDIN_FILENO or STDOUT_FILENO. It specifies standard file descriptor that is to be replaced with fd.

• What is return value use for?

Use of dup2#include <stdio.h>#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <unistd.h>

void main(void){ int fd; mode_t fd_mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH; if ((fd = open("my.file", O_WRONLY | O_CREAT, fd_mode)) == -1) perror("Could not open my.file"); else { if (dup2(fd, STDOUT_FILENO) == -1) perror("Could not redirect standard output"); close(fd); } printf("This is a test\n");}

File Descriptor Table

file descriptor table after open

[0] standard input

[1] standard output

[2] standard error

[3] write to my.file

file descriptor table after dup2

[0] standard input

[1] write to my.file

[2] standard error

[3] write to my.file

file descriptor table after close

[0] standard input

[1] write to my.file

[2] standard error

pipe

pipe(fd)• fd is array of size 2• Unidirectional. Read from fd[0], write to

fd[1]• System V release 4 implements bi-

directional communication mechanisms called STREAMS.

• Return value?

pipe Example/* Example 3.20 */#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <fcntl.h>void main(void){ int fd[2]; pid_t childpid; pipe(fd); if ((childpid = fork()) == 0) { /* ls is the child */ dup2(fd[1], STDOUT_FILENO); close(fd[0]); close(fd[1]); execl("/usr/bin/ls", "ls", "-l", NULL); perror("The exec of ls failed"); } else { /* sort is the parent */ dup2(fd[0], STDIN_FILENO); close(fd[0]); close(fd[1]); execl("/usr/bin/sort", "sort", "-n", "+4", NULL); perror("The exec of sort failed"); } exit(0);}

pipe after forkParent

file descriptor table

[0] standard input

[1] standard output

[2] standard error

[3] pipe read

[4] pipe write

Child

file descriptor table

[0] standard input

[1] standard output

[2] standard error

[3] pipe read

[4] pipe write

pipe

child

parent

[2]

[0]

[4][3]

[1][0]

[1]

[2]

[3] [4]

pipe after dup2Parent

file descriptor table

[0] read pipe

[1] standard output

[2] standard error

[3] pipe read

[4] pipe write

Child

file descriptor table

[0] standard input

[1] write pipe

[2] standard error

[3] pipe read

[4] pipe write

pipe

child

parent

[2]

[0]

[4][0]

[1]

[2]

[3] [1]

[3]

[4]

pipe after closeParent

file descriptor table

[0] read pipe

[1] standard output

[2] standard error

Child

file descriptor table

[0] standard input

[1] write pipe

[2] standard error

pipe

child

parent

[2]

[0]

[0]

[1]

[2]

[1]

Reading/WritingSYNOPSIS

#include <unistd.h>

ssize_t read(int fildes,void *buf, size_t nbytes);

POXIX.1, Spec 1170

read nbytes from fildes

****************************************

SYNOPSIS

#include <unistd.h>

ssize_t write(int fildes,const *buf, size_t nbytes);

POXIX.1, Spec 1170

write nbytes to fildes

Blocking I/0 with Pipes

• Empty buffer is not an end-of-file indication• When a process attempts to read an

empty buffer, it blocks• If the process wants to read from two

different buffers, it could attempt read an empty buffer first and hang indefinitely

POSIX Required SignalsSymbol Meaning

SIGABRT

SIGBUS

SIGALRM

SIGFPE

SIGHUP

SIGILL

SIGINT

SIGKILL

SIGPIPE

SIGQUIT

SIGSEGV

SITTERM

SIGURG

SIGUSR1

SIGUSR2

Abnormal termination as initiated by abort

Access undefined part of memory object

Timeout signal as initiated by alarm

Error in arithmetic operation as in division by zero

Hang up (deat) on controlling terminal (process)

Invalid hardware instruction

Interactive attention signal

Terminate (cannot be caught or ignored)

Write on a pipe with no readers

Interactive termination

Invalid memory reference

Termination

High bandwidth data available at a socket

User-defined signal 1

User-defined signal 2

POSIX Job Control Signals

Symbol Meaning

SIGCHLD

SIGCONT

SIGSTOP

SIGTSTP

SIGTTIN

SIGTTOU

Indicates child process terminated or stopped

Continue if stopped (done when generated)

Stop signal (cannot be caught or ignored)

Interactive stop signal

Background process attempts to read from controlling terminal

Background process attempts to write to controlling terminal

kill System Call Example#include <stdio.h>#include <sys/types.h>#inlcude <signal.h>

if (kill(3423, SIGUSR1) == -1)perror(“Could not send SIGUSR1”);

if (kill (getppid(), SIGTERM) == -1)perror(“Error in kill”);

raise System CallSYNOPSIS

#include <signal.h>int raise(int sig);

POSIX:CX

A process can send a signal to itself with a raise system call – Example:

if (raise (SIGUSR1) != 0)perror (“Failed to raise SIGUSR1”);

sigaction System CallSYNOPSISint sigaction(int signo, const struct sigaction *act, struct sigaction

*oact); struct sigaction {

void (*sa_handler)(); /* SIG_DFL, SIG_IGN, or pointer to function */sigset_t sa_mask /* additional signals to be blocked during

execution of handler */int sa_flags /* special flags and options */

The sigaction system call installs signal handlers for a process. The data structure struct sigaction holds the handler information

sigaction – Example#include <signal.h>#include <stdio.h>#include <sys/types.h>#include <string.h>char handmsg[] = “I found ^c\n”;

void mysighand(int signo){ write (STDERR_FILENO, handmsg, strlen(handmsg)); }void main() {struct sigaction newact;… ; newact.sa_handler = mysighand; /* set the new handler */sigemptyset(&newact.sa_mask); /* no other signals blocked */newact.sa_flags = 0; /* no special options */if (sigaction(SIGINT, &newact, NULL) == -1)

perror("Could not install SIGINT signal handler");… ; }

Installs mysighand signal handler for SIGINT